As for the recording method by means of an inkjet printer which is one of the typical methods among various color recording methods, ink droplets are generated and adhered onto various record-receiving materials (such as paper, film and cloth) to perform recording. This method has been rapidly prevailing lately and is expected to continue growing remarkably in the future because of features such as quietness with a little noise generation due to no direct contact of a recording head with a record-receiving material and as easiness in downsizing and speeding up. Conventionally, as an ink for fountain pens, felt-tip pens or the like and as an ink for inkjet recording, inks where a water-soluble dye is dissolved in an aqueous medium have been used. In these water-based inks, a water-soluble organic solvent is generally added to prevent ink from clogging at a pen tip or an inkjet nozzle. These inks are required to provide recorded images with sufficient density, not to clog at a pen tip or a nozzle, to dry quickly on a record-receiving material, to bleed less, to have excellent storage stability, and so on. In addition, the water-soluble coloring matter to be used is required to have high solubility particularly in water and in water-soluble organic solvents to be added to the inks. Further, recorded images formed are required to have fastnesses such as water fastness, light fastness, ozone gas fastness and moisture fastness.
The above described ozone gas fastness means durability against the phenomenon that ozone gas existing in the air and having oxidizing effects acts on coloring matter in recorded paper, leading to discoloration or fading of printed images. Besides ozone gas, the oxidizing gas having this kind of action includes NOx, SOx and the like. However, among these oxidizing gases, ozone gas is regarded as the main causative matter which drives the discoloration or fading phenomenon of inkjet-printed images.
In many ink receiving layers to be provided on the surface of special paper for photo quality inkjet, materials such as porous white inorganic substance are used in order to dry ink sooner and to reduce ink bleeding on high image quality. On such recorded paper, discoloration or fading caused by ozone gas is prominently observed. Discoloration or fading phenomenon caused by this oxidizing gas, particularly by ozone gas, is characteristic on inkjet images, so improvement of ozone gas fastness is one of the most important problems in inkjet printing method.
In order to expand the application field of the printing method using ink in the future, ink compositions to be used for inkjet printing and colored products colored therewith are strongly desired to have light fastness, ozone gas fastness, moisture fastness and water fastness which are further improved.
Inks having different hues are prepared from different coloring matters, and among them, black ink is an important ink to be used for both mono- and full-color images. Many coloring matters have been proposed as a coloring matter for these black inks, but any of them has yet to provide a product sufficiently satisfying the market requirements. Many of the proposed coloring matters are azo coloring matter. Among them, disazo coloring matter such as C.I.Food Black 2 has problems such as low optical density of image, poor water and moisture fastnesses, and insufficient light and gas (ozone) fastnesses. Polyazo coloring matter where the conjugated system is extended has generally problems such that water-solubility is low, bronzing phenomenon that recorded image partially has metallic luster is easily caused, and such that its light and gas fastnesses are not sufficient. Further, among many metal-containing azo coloring matters proposed, some have good light fastness, but they similarly have problems such that they are not desirable in terms of biological safety and environmental problems because it contains metal ion, and in addition to that, they are also extremely low in ozone gas fastness.
Black coloring matter for inkjet improved in ozone gas fastness which is the most important problem in recent years includes, for example, the compounds described in Patent Literatures 1 to 3. These compounds do not have ozone gas fastness satisfying the market requirements or sufficient light fastness. In addition, Patent Literatures 4 to 7 describe azo compounds having a benzimidazolopyridone structure in a molecule. Patent Literature 4 discloses a monoazo or disazo coloring matter compound which is soluble in organic solvents. Patent Literature 5 discloses a trisazo compound. These trisazo compounds are a compound having a symmetrical structure where the both ends of the linking group comprising a trisazo structure bear two benzimidazolopyridone structures. Further, none of Patent Literatures 4 to 7 discloses use examples of the black compounds for inkjet ink. Examples of a black trisazo compound for inkjet not having a benzimidazolopyridone structure and having ozone gas fastness include the compound described in Patent Literature 8.    [Patent Literature 1] JP 2003-183545 A    [Patent Literature 2] JP 2003-201412 A    [Patent Literature 3] JP 2007-517082 A    [Patent Literature 4] WO 2004/050768    [Patent Literature 5] DE Patent 2004488    [Patent Literature 6] DE Patent 2023295    [Patent Literature 7] JP H05-134435 A    [Patent Literature 8] WO 2005/054374